You said it's spring there so I'm assuming you mean 20-25 Celsius correct? If you mean Celsius then those are not too cold of temperatures for a brooder to be with a supplemental heat source in fact any hotter and I would be worried about overheating using a heat lamp unless you keep it far enough away from the brooder and are not using a red bulb. The entire brooder should not be heated only a corner where your duckling can go to warm up if needed, but still get out of the heat when hot. Trying to heat the whole brooder will overheat your duckling. Do you have a thermometer that you can place directly under the heat lamp? It's best to always keep a thermometer under your heat lamp when using it in a brooder because you can very easily overheat a duckling with a lamp (red bulbs especially). The best way to tell how your duckling is adjusting to the temperature is to see where they sit in the brooder. A duckling will sit directly under the heat when the brooder is cold, a duckling that is in a brooder with correct temperatures will sit just on the edge of the heat and a duckling that is overheating will sit in the far corners away from the heat.